The present invention relates to electrical plugs; more particularly, the invention relates to heavy duty electrical plugs in the 30, 60 and 100 amp range which will grip and seal electrical cords of different diameters.
Heavy duty electrical plugs are used to connect high current, portable electrical equipment to electrical receptacles. The plugs provide for termination of an electrical cord into a number of prongs which are mated to the receptacle. The plug assemblies usually consist of a body, a cover secured to one end of the body where the electrical cord enters and a plug sleeve attached to the opposite end of the body where the prongs are located. Typically, the size of the plug assemblies varies between 30, 60 and 100 amp applications.
Because the electrical equipment with which these plugs are used is generally portable and is moved around a worksite, the plugs must be able to withstand pulling without the cord separating from the plug. Certain pulling force standards have been established by Underwriters Laboratories for these plugs. For example, a 30 amp plug must withstand a pull of 150 pounds whereas a 60 or 100 amp plug must withstand a pull of 300 pounds. To satisfy the standard pull tests, gripping devices are used to secure the cord to the plug body.
The problem of cord gripping is made more challenging by cords of different diameters which are used in the same size plug assemblies. For example, in the 60 amp range, the outside diameter of a cord can vary between 3/4" and 1.4" Therefore, the cord gripping arrangements of each plug must be adjustable to accommodate electrical cords of various diameters.
Various arrangements presently exist for securing cords of different diameters to the body of a heavy duty plug. In one arrangement, the gripping mechanism consists of an adjustable, two-jaw clamp, externally mounted at one end of the plug where the cord enters the plug body. U.S. Pat. No. 3,739,318 discloses such an external clamp. In a hostile environment, this external clamping arrangement is subject to damage and the adjusting screws on the clamp can loosen and become lost. In a more recent clamping arrangement, the two-jaw clamp is enclosed in a cover. However, the adjusting screws used to tighten the clamp are located outside the cover exposing them to possible damage. An additional problem with all two-jaw clamps is that they grip in only two directions, causing the circular cord to become deformed and flattened. U.S. Pat. No. 4,874,332 provides for an internal adjustable clamp, but the clamp is for light duty use and secures individual conductors only; not the main cord. There is, therefore, a need for a cord grip which can grip a variety of different sizes of electrical cords, but is protected by being placed within the plug body.
In addition to the need for gripping cords of varying sizes, plugs must also provide an environmental seal to prevent moisture, dirt and fumes from entering the plug body. In prior art plug assemblies, users have been provided with bushings of different sizes. A bushing is then chosen for the particular cord to be used. Unused bushings are then discarded thus creating unnecessary waste and contributing to the pollution of the environment. If the plug is to be reused with an electrical cord of a different diameter, the proper size bushing may be lost and the plug will then be used with an improperly sized bushing or even without a bushing.
There is a need therefore, for an electrical plug body which will both internally secure an electrical cord of varying diameters and provide an internal environmental seal around electrical cords of varying diameter without the need for additional bushings. Such plugs should also be easy to use yet provide for secure mounting to electrical cords.